1. Field of the Invention
For the past few years there has been interest in transmitting and receiving high-speed data over telephone wires to individual subscribers without having to provide higher-quality cables. High speed in this context means rates such as 1.5 Mb/s (U.S.A.), 2 Mb/s (Europe). Two systems have emerged:
1) High-speed digital subscribers line (HDSL). The equipments at each end of the line are nominally indentical. They transmit and receive data simultaneously at the same rates, i.e., full duplex. It is expected that two or three lines will be needed to achieve the desired capacity, each signalling at 50% or 33% respectively relative to a single-line implementation.
2) Asymmetric digital subscribers line (ADSL). On the basis that most subscribers do not have the means or the need to generate such large amounts of data, the channel from subscriber to exchange is reduced to a low-speed signalling circuit, typically 16 kb/s, while retaining a high-speed channel from exchange to subscriber. An obvious application for this is the transmission of a selectable broadcast-quality video signal. An ADSL system has a much longer transmission range than an HDSL system of the same baud rate as will be discussed later.
Transmission standards are still under discussion. For HDSL the main contenders are pulse-amplitude modulation, probably 2B1Q as used on ISDN, or Quadrature Amplitude Modulation (QAM) (and a recent variant termed carrierless amplitude/phase modulation (CAP). For ADSL it is most likely that the American National Standards Institute (ANSI) will specify discrete multiple tone (DMT).
DMT is fully described in tutorial paper T1E1.4/91-157 "A Multicarrier Primer" J. M. Cioffi, Amati Communications Corporation presented to Standards Committee T1-Telecommunications, of ANSI and imported herein by reference, but its basic principle is as follows:
Successive blocks of the high-speed data stream are demultiplexed into a large number of channels (typically 256). Each channel may be several bits wide and different channels may be of different widths. Each channel is then QAM-modulated on to one of 256 cariers. The carrier frequencies are all harmonics of the lowest one. This composite spectrum is transmitted for the duration of the data block length. Viewed another way, each subset of bits within the data block describes the amplitude and phase of a tone burst at a frequency specific to that group. The tone burst lasts for a data-block duration, and the tones are a simple harmonic series (FIGS. 1 & 2).
The claimed advantages of DMT lie not so much in its transmission performance or distance under normal conditions but in its implementation and flexibility: (a) the composite spectrum is generated and decoded by a fast Fourier transform (FFT) and its inverse, without the need for generating individual carriers; (b) since each carrier occupies a small bandwidth it suffers little frequency distortion so removing the need for complicated line equalizers; (c) by monitoring the error rate of each carrier, the receiver can instruct the transmitter to preferentially use those parts of the spectrum with the best performance.
HDSL is limited by near-end crosstalk (NEXT). To the attenuated signal from the distant transmitter is added an unwanted signal that originates from transmitters physically close to the receiver and which is coupled via inter-wire capacitance. With conventional coding, the wanted signal and the NEXT occupy the same frequencies. The problem is greater at the exchange end where lines are concentrated than it is at the subscriber's end.
With ADSL the subscriber's receiver only receives NEXT from low-speed sources so the limiting factor is noise from unwanted high-speed transmitters at the exchange (far-end crosstalk). The noise source is thus the same as for HDSL but the noise is attenuated by the line given an ADSL system better Signal/Noise ratio (SNR).
In full-duplex operation the received signal may contain an element of the signal transmitted on the same wire-pair that is caused by impedance discontinuities in the transmission medium or imbalance in a hybrid circuit. In many data systems it is necessary to include an echo canceller to remove this interfering signal.